The present invention relates to apparatus and methods for precision slewing, or pointing, a platform-mounted device, such as a camera. More particularly, this invention pertains to the slewing of a two-axis gimbal-mounted device with mechanical gyro stabilization.
Two degree-of-freedom gyros have been commonly employed in the prior art to maintain or stabilize the orientations of the axes of platforms, cameras and other devices with respect to the earth or inertial space. Such gyros are characterized by a mass (xe2x80x9crotorxe2x80x9d) that rotates is about, and thereby defines, a spin axis. Generally the device to be stabilized is fixed to the case of the gyro so that any deflection of the position of the device with respect to a stabilized axis is sensed as movement of the attached case of the gyro with respect to the stabilized and spinning rotor. This generates a corrective signal that is transmitted to a platform-fixed motor for generating a corrective force with respect to the temporarily-misaligned axis.
In many cases it is also important to be able to slew a device (such as a camera) about some preferred axes to change its orientation in space. The standard technique for doing this is to precess the gyro spin axis about the appropriate output axis. Pickoffs internal to the gyro detect the precession and coerce the supporting gimbals to follow it thereby changing the orientation of the device in space.
If the gimbal axes are aligned to the gyro axes, the slewing can be controlled about the desired axis without cross-coupling motion into the other axis. This result is only true in the prior art for steady state rates of slewing. However, in the case of low frequency oscillatory slewing (i.e. xe2x80x9cscanningxe2x80x9d), because the gyro dynamics includes the inertia of the gyro rotor, the gyro torquers tend to deflect the gyro spin axis about both the azimuth and pitch axes when only one axis is desired to be sinusoidally precessed. As a result, the rotor spin axis is caused to cone in an undesirable elliptical manner.
The present invention addresses the foregoing shortcomings of the prior art by providing, in a first aspect, apparatus for controlling the path of oscillatory travel of a device within a two-axis system in which the device is fixed to a two degree-of-freedom gyroscope. The gyroscope includes a first forcer for applying a torque with respect to a first rotor axis in response to a first signal to precess the rotor about a second, orthogonal rotor axis. It includes a second forcer for applying torque to the rotor with respect to the second rotor axis in response to a second signal. The angular displacement of the rotor from a null position generates a signal for activating motion to position the device within the two-axis system.
The apparatus of the invention includes at least one cross-axis circuit arranged to receive the first signal and to generate the second signal in response so that the said second signal drives the second forcer to precess the rotor with respect to the first axis to substantially cancel the effect of torque applied by the first forcer with respect to the first axis of the rotor.
In a second aspect, the invention provides apparatus for substantially nulling the effect of torque applied to precess the spinning rotor of a gyroscope. Such apparatus includes a first forcer for applying a torque with respect to a first axis of the rotor in response to a first signal. A second forcer is provided for applying a torque to the rotor with respect to a second axis in response to a second signal. The second axis is orthogonal to the first axis.
A cross-axis circuit receives the first signal and generates the second signal in response so that the second signal drives the second forcer to apply torque to the rotor with respect to the first axis by precession to cancel the torque applied to the rotor with respect to the first axis by the first forcer.
In a third aspect, the invention provides a method for substantially nulling the effect of a first torque applied by a first forcer with respect to a first axis of a spinning gyroscope rotor to precess the rotor with respect to a second, orthogonal, axis of the rotor. Such method comprises the step of applying a second torque to the rotor with respect to the second axis of the rotor to precess the rotor with respect to the first axis to substantially cancel the effect of the torque applied to the rotor with respect to the first axis by the first forcer.
The preceding and other features of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the written text and the drawings.